The invention relates to an inductively coupled or capacitively and inductively coupled source for a plasma processing system, such as, for example, a plasma processing system of the type used in the manufacture of electronic components.
Referring to FIG. 1, an inductive plasma source may consist of a single section inductor (e.g. a coil 10) within or adjacent to a vacuum chamber 12, which may be composed of dielectric, semiconducting, and/or conducting materials. Coil 10 transfers power to a gas or mixture of gases within vacuum chamber 12 that is held at a low absolute pressure. In a conventional system of this type, RF power is typically delivered to the coil through one terminal of coil 10 while the other terminal of coil 10 is connected through a capacitor 14 to ground.
In the general case, power is transferred into the chamber via electric field and magnetic field coupling. A plasma of ionized gas is induced in the chamber when the field strength developed by coil 10 is sufficient to excite many electrons beyond the ionization energy of one or more of the gas constituents. For an inductive plasma, the magnetic field provides the dominant mode of power transfer.
The magnetic field created in coil 10 is determined by the current through the coil and the mechanical design of the coil. Generally, for inductive plasma sources, it is desirable to achieve the maximum magnetic field and therefore the maximum coil current for a given input power level.
Inductive sources also have an electric field (capacitive) mode of coupling related to the inductor voltage, unless it is blocked by some mechanism such as a Faraday shield. The voltage across an inductor is defined by the generalization of Ohm""s law applied to an element which may be considered a lumped inductance:                     v        =                  L          ⁢                                    ⅆ              i                                      ⅆ              t                                                          (        1        )            
That is, the voltage across an inductance is proportional to the time rate of change of the current through the inductance. The constant of proportionality L is the value of the inductance, given in units of henries. Thus, the voltage gradient across an inductor is set by the mechanical factors which determine the inductance, the rate of change of current in the coil set by the spectrum of the driving function, and the maximum coil current.
Maximizing the coil magnetic field for a given power input depends upon two conflicting factors, namely, the current through the coil and the inductance of the coil. The inductance of a coil at a given frequency f can be written as follows: z=jxcfx89L, where z is a complex impedance, j=(xe2x88x921)xc2xd, xcfx89=2xcfx80f (i.e., the radian frequency of the driving function), and L equals inductance. Since the impedance of a coil at a particular frequency is proportional to the inductance, a small inductance would maximize the current through the coil. However, the magnetic field in a coil is increased by increasing the inductance. The solution is to use the concept of circuit resonance such that the coil is part of a tuned circuit with the desired characteristics at the frequency of interest.
The principle of electrical resonance is based upon the energy storage capability of inductances and capacitances. The fact that energy storage in a capacitance is in an electric field, and energy storage in an inductance is in a magnetic field results in a 180xc2x0 phase shift between voltages across each element in a series circuit, resulting in a low impedance across the circuit at resonance. In a parallel circuit at resonance, the currents through the elements are out of phase, resulting in a high impedance across the circuit. A combination of series and parallel elements can result in the desired impedance match of the source inductor to the driving generator for maximum current.
The inductor peak voltage relative to the plasma potential establishes an electric field in the plasma which may result in the acceleration of the charged species alternately toward and way from the inductor or the walls of the vacuum vessel. These species may impact and sputter the wall of the chamber resulting in physical damage in the form of pitting, which is generally referred to as worm-holing, reducing the life of expensive components. Sputtered material from the walls, or the wall itself, may also become involved in a chemical reaction which may influence chemical processes within the vacuum vessel, producing possibly undesirable results.
In addition, in conventionally designed systems there may be arcing between the high voltage terminal at the top of the coil and the grounded surfaces. The arcing problem may not be solvable by simply moving the high voltage side of the coil to the other terminal. That could merely move the arcing problem to the other end of the chamber where other ground metal surfaces exist close by. Moreover, such a significant alteration in the system (i.e., moving the high voltage terminal of the coil antenna) may alter the process environment in an unpredictable and undesired way.
Also, it may not be possible to solve the arcing problem by simply moving the high voltage terminal farther away from the silicon plate or the other grounded conductive parts. The electric field that is created in that region of the system is important plasma ignition. If the high voltage turn of the coil was moved farther away from the metal part, the electric field would be reduced and initiating the plasma would become more difficult.
The invention is a plasma processing system including, in its simplest form, a series circuit made up of a first capacitor, a second capacitor, and a coil (inductor) used to generate an inductive plasma in a vacuum vessel. The particular topology allows the voltage difference between each end of the inductor and the ground reference to be shifted. If the capacitors are variable capacitors, the amount of shift can be adjusted by varying the values of the capacitances. If the plasma potential is referenced to ground, the inductor voltage relative to the plasma potential may be controlled.
In general, in one aspect, the invention is a plasma system which is coupled to a high frequency power source. The plasma system includes a chamber defining an internal cavity in which a plasma is generated during operation; a coil which during operation couples power from the power source into the plasma within the chamber; a first capacitor through which a first terminal of the coil is coupled to a reference potential; and a second capacitor through which a second terminal of the coil is coupled to the power source.
Preferred embodiments include the following features. The coil is positioned outside of the internal cavity within the chamber. The power source operates at a frequency of f, the coil has an inductance of L when the plasma is present in the chamber, the first and second capacitors have capacitances of C1 and C2, respectively, the combination of the first and second capacitors and the coil forms a circuit characterized by a resonant frequency that is (2xcfx80)xe2x88x921(LC)xe2x88x92xc2xd where C is equal to:       1                            (          C1          )                          -          1                    +                        (          C2          )                          -          1                      ,
and said resonant frequency is selected to be near f. In addition, the chamber includes a dome made of a dielectric or semiconductor material and the coil surrounds the outside of the dome. The plasma system further includes a matching network connected between the power source and the second capacitor. Also, the reference potential is a ground potential. Further, the second capacitor has first and second terminals and the second terminal of the second capacitor is connected to the second terminal of the coil and the plasma system includes a shunt capacitor connected between the first terminal of the second capacitor and a second reference potential (e.g. a ground potential).
Also in preferred embodiments, both the values of the first and second capacitors are selected to satisfy the following relationships:   C2  =            (              ρ                  ρ          -          1                    )        ⁢    C  
wherein xcfx81 greater than 1 and C≈(Lxcfx892)xe2x88x921, where L is an inductance of the coil with a plasma present and xcfx89 is a radian frequency of the power source. Furthermore, at least one of the first and second capacitors is a variable capacitor.
The invention keeps the voltage across the coil approximately the same as compared to the conventional approach but it enables the user to shift that voltage relative to ground potential.
This has been shown to be useful in eliminating worm holing in the dome near the high voltage terminal of the coil antenna. In other words, it offers a substantial benefit in terms of extending the life of the quartz domes that are used in etch chambers.
It has also been found to provide an additional way of controlling the physical and/or chemical reactions within the plasma chamber and thus giving the user further ability to tailor the process conditions to the particular needs of each application.
The invention significantly reduces likelihood of arcing to the grounded surfaces. It also reduces the voltage gradient between the plasma and the coil. This results in less bombardment of the quartz cylinder by ions that are generated during the processing. Thus, another direct benefit of the invention is a considerably longer life of the quartz cylinder, which can typically cost thousands of dollars.
Other advantages and features will become apparent from the following description of the preferred embodiment and from the claims.